1. Field of the Invention
The present invention relates to a system for detecting the position of a break or rip in a previous water-barrier sheet placed in a waste disposal facility for isolating wastes dumped over the water-barrier sheet and the ground or soil of the waste disposal facility.
2. Description of the Related Art
Waste disposal facilities are constructed as landfill sites for the purpose of safe storage of wastes with no hindrance to preservation of our environment via stabilization and conversion to harmless materials using metabolic actions of the natural world. The wastes are generally classified into two groups according to their origin or source, namely, non-industrial wastes and industrial wastes. Aside from the classification, our activities in homes and industries deliver a wide variety of waste materials containing organic substances, such as kitchen refuse, ligneous waste, paper rubbish, fibrous waste, and organic sludges. Particularly for an organic waste containing harmful constituents, the in-ground disposal requires a careful control against a possible contamination to our environment caused due to degradation, elusion or the like transformation of the buried organic waste.
In general, wastes buried in a controlled industrial waste final disposal facility or a non-industrial waste disposal facility are decomposed or degraded by microbes as the time goes on. The buried wastes, however, still have a possibility of generating hazardous substances via chemical reactions, posing a contamination to the proximal environment including ground water and other public water resources when leaking with rain water, for example. To deal with this problem, the waste disposal facilities normally have a water-barrier sheet of synthetic resin or synthetic rubber placed for isolating the buried wastes from the ground or soil of the waste disposal facility.
The water-barrier sheet, when ruptured or otherwise damaged due, for example, to a defect in the joint structure, insufficient smoothness of the excavation surface, or an inadvertent operational error during waste dumping operation, will allow hazardous constituents in the waste materials to leak into the ground water, posing the danger of contamination to the water supply and agricultural water resources. Accordingly, early detection and subsequent mending of a break in the water-barrier sheet is of great importance to the waste disposal facility.
Various methods have been proposed for detecting leakage water from the waste disposal facility. One such known leakage detection method includes a ground-water monitoring well which is drilled in the proximity of the waste disposal site for evaluating the quality of ground water. This method, however, is unable to identify the position of leakage, and accordingly it takes a long time to detect and mend the broken part of the water-barrier sheet.
In another known method, an electric proving or surveying technique is used to detect the position of a break in the water-barrier sheet. Typical examples of such electric surveying technique are known as a potential measuring method and a current measuring method. According to the potential measuring method, a voltage is applied in and around the waste disposal site, so that the position of a leak in the water-barrier sheet can be detected from a distortion in potential distribution appearing inside the waste disposal site. On the other hand, the current measuring method utilizes upper and lower arrays of regularly spaced linear electrodes disposed crosswise on opposite sides of a water-barrier sheet to detect the position of a leak in the water-barrier sheet in terms of a change in current flowing through the electrodes.
These known electric methods, however, are low in reliability due, for example, to expert knowledge required for analyzation of data obtained, numerous uncertain factors involved, such as an underground current and a leakage current, and high susceptibility to external disturbances. Another drawback is in a low adaptability to variable factors such as aging of the buried waste materials. Accordingly, the known electric methods are unable to realize an accurate delay-free detection of a broken part of the water-barrier sheet in the waste disposal facility.
Japanese Patent Laid-open Publication No. 4-168335 discloses a leak monitoring system used in a pipe line, such as a water supply line, for monitoring a possible leak using an optical fiber laid alongside the pipe line as a temperature sensor. In general, the optical fiber when heated locally, the heated portion will generate greater quantities of scattered light than the rest or unheated portion. Taking this phenomenon into consideration, it may be resumed that a measurement of back scattered light coming back from the heated portion to one end of the optical fiber while a pulsed light beam is emitted from the same end will make it possible to determine the temperature of the heated portion according to the intensity of the back scattered light, and the position of the heated portion according to a time period between the emission of the pulsed light beam and the arrival of the back scattered light. According to the disclosed leak monitoring system, an optical fiber is laid alongside the pipe line in thermally isolated condition and at a position ready to be flooded with leakage water from the pipe line, with one end of the optical fiber connected to a back scattered light measuring instrument.
With this arrangement, when a portion of the pipe line is immersed in or flooded with leakage water from a break in the pipe line, the flooded pipe portion has a different temperature than the rest of the pipe line. Accordingly, by measuring the back scattered light of the optical fiber using the back scattered light measuring instrument, the temperature and position of the leakage water and hence the position of the break in the pipe line can be detected.
However, there is no teaching in Japanese Patent Laid-open Publication No. 4-168335 that the disclosed leak monitoring system can be used in a waste disposal facility for the purpose of detecting the position of a break in a water-barrier sheet placed in a waste disposal facility for isolating wastes from the ground or soil of the waste disposal facility. In practice, the disclosed leak monitoring system is not readily applicable to the detection of a broken part of the water-barrier sheet in the waste disposal facility to which the present invention pertains.
Japanese Patent Laid-open Publication No. 4-294236 discloses a method of detecting the position of a flooded portion of an optical fiber composite overhead-earth line, in which water penetrating into the optical fiber composite overhead-earth line can be detected by an optical fiber temperature distribution sensor in terms of a change in temperature caused by an exothermic reaction or an endothermic reaction between the penetrating water and a material which is capable of generating or absorbing heat when reacted with water. Typical examples of the exothermic material are calcium chloride (CaCl.sub.2) and ferrous chloride (FeCl.sub.2).
The detection of a broken part in the pipe line such as disclosed in Japanese Patent Laid-open Publication No. 4-294236 is far distant from the detection of a broken part of a water-barrier sheet placed in a waste disposal site for isolating wastes and the soil of the waste disposal facility. The disclosed detecting method, like the method disclosed in Japanese Patent Laid-open Publication No. 4-168335 discussed above, is not readily applicable to a breakage detection system for a water-barrier sheet in the waste disposal facility to which the present invention pertains.